Method of locating beacons



June 7, 1949. E, HUDSPETH 2,472,212

METHOD oF LocATING BEAcoNs Filed July,- 1945 FIG.| FIG'. 2

FIG.4

. INVENTOR. EMMETT' L. HUDSPETH A TTOR/VEV clutter.

Patented June?, 1949 UNITED STATES PATENT OFFICE Emmen L. Haspeth, cambridge, Mass., assigner, by mesne assignments, to the United States of America' as represented by the Secretary of-War Application July 9, 1945, Serial No. 604,027

2 claims. l

This invention relates to energy beam reflectors, and more`particularly to reflectors formed by two flat sheets of electrically conductive material which meet at an angle to form a corner. Such a reflector, hereinafter referred to as a diplane is 'adapted to reflect an impinging beam of radiant energy back toward its source in such a manner as to impart a readily identifiable characteristic to the reflected beam. Thus a reflector of this character may be utilized as a marker for use` with a radio object-locating system.

In its most general aspect, the diplane herein comprises two plane metallic sheets disposed at an angle (90 or less) with each other to form a corner. Incident -wave energy travelling in a plane of propagation bisecting the corner angle is directively reflected in the same plane. If the incident energy is polarized in the above-mentioned plane of propagation, or at right angles thereto. there is no shifting of the plane of polarization of the energyv on reflection. However, if the energy is polarized at an angle of 45 tothe plane bisecting the corner angle, it is reflected with a 90 shiftin polarization.

These characteristics make a diplane particularly advantageous as a target marker for use in conjunction with a radio object-locating system, such as for returning av strong signal to an aircraft or ship having a transmitting and receiving antenna adapted to transmit and receive either vertically or horizontally polarized energy,

and to transmit vertically polarized energy and receive horizontally polarized energy or vice versa.

Furthermore, a signal returned by a diplane is easily distinguishable from the depolarized energy (having anv angular shift of polarization) returned by land and sea targets and what is included in the terms ground clutter and sea It is the principal object of the present invention to provide a simple and inexpensive device vwhich is adapted to reflect a highly directional beam of energy which is not depolarized on reflection.' A. f

It is another object of the invention to provide a reflector adapted to reflect a beam of incident energy and to shift the plane of polarization of the incident energy under certain conditions by a predetermined angle, preferably of 90.

Other objects and advantages of the invention 2 will become apparent as the description proceeds.

In the drawings:

Fig. 1 is a 'side elevation view of the diplane and supporting structure according to one embodiment of the present invention;

Fig. 2 is a -top plan view of the same;

Fig. 3 is a front elevation of the diplane; and

Fig. 4 is a polar diagram showing the intensity of .a signal rreturned by the diplane of Figs. 1, 2, and 3 as a function of the angle through which the diplane is rotated.

Referring to Fig. 1, there is shown `a diplane I0-` comprising two plane reecting members or sheets I I and I2 meeting at an angle along a line or common boundary I3. The members I I and I2 may be perpendicular to each other, or the angle a formed by their reecting surfaces may be more although preferably less, than The reflecting surfaces of members II and I2 are of appropriate character for the efcient reflection of one or several desired-types of radiant energy, more particularly energy having wavelengths of the order of centimeters.

The size and character of the reecting surfaces of members II and I2 are of course determined by the kind or kinds of energy beams it is desired to reflect. The general rule for eicient reflection is that the dimensions of the surface should be large, and irregularities in the surface should be small (i. e., the surface should be smooth) as compared to the wavelength of the energy to be reflected. Thus, for' short Waves highly polished surfaces are desirable, while for longer waves, av

course mesh of copper wire or other conducting material may be entirely satisfactory.

The diplane I0 is supported in any suitableb manner, such as by an upstanding pedestal I4.

means of rotating diplane Il is shown by way of example in Fig. 1 in which a shaft II is mounted near the upper end of pedestal Il in any suitable manner so that shaft Il is free to rotate about its-axis, the axis being generally perpendicular to the longitudinal axis of pedestal Il. One end of shaft I5 is secured to a rear portion of diplane I5, preferably at the approximate center of line I3. Any suitable means may be provided for rotating diplane I such as by motor IB having driving connection, through belt I'I, to a pulley I8 fixed on shaft I Ii.

' For purposes of facilitating illustration it may be assumed that diplane I0 is an isolated beacon normally in a vertical position as shown in Figs. 1 and 3 and capable of rotation at a constant speed about its axis It is also assumed that a radioobject locating system is adapted to transmit and receive high frequency waves of electromagnetic energy'which impinge on diplane III in a plane bisecting the dihedral angle a formed by members II and I2. In the vertical position of diplane I0, vertically polarized incident energy will be reflected Without polarization change and a strong reflected signal will be received by the receiving apparatus of the system. As diplane III rotates about axis .'r, the strength of the received signal varies with the angular position of diplane I Il as illustrated in solid lines in Fig. 4, the signal strength being a maximum for the horizontal and vertical positions of diplane I0 corresponding to the perpendicular and parallel positions of line I3 relative to the plane of polarization of the incident energy. This holdstrue where the incident energy is either horizontally or vertically polarized and the transmitting and receiving apparatus have antennas adapted to transmit and receive energy of the same polarization.

If, however, the transmitting apparatus is vertically polarized and the receiving apparatus is horizontally polarized or vice versa. the solid line configuration of Fig. 4 is shifted by 45 as shown by the pattern in broken lines. This is due to the fact that, as diplane III is rotated through an angle 0 with the horizontal, the electric vector is rotated through an angle 20. Thus when the incident wave is transmitted vertically, and when 0=45, the electric vector of the return wave is horizontal. For this reason, the plane of polarization is shifted 90 when the diplane is at angles of 45 relative to the plane of polarization of the incident energy.

By rotating diplane I0 about axis z, the signal returned thereby to, and shown on an indicator of, a radio object-locating apparatus will fluctuate in intensity at a frequency four times that of the rate of rotation of diplane I0. In this way, an operator of a radio object-locating system, located for example in an airplane, can readily observe theposition of the point marked by diplane I0 by comparison with other fixed signals such as may be returned by wooded areas, buildings, or other fixed objects. In view of this, one particularly useful application of this invention would be in the marking of a runway for an aircraft landing iield.

According to another embodiment of the invention diplane I0 may be mounted as described hereinbefore, but in fixed angular relation;

rather than rotatable, to its supporting means;

Thus diplane III may be positioned with line I3 forming an angle 0=45 to the horizontal plane. With the diplane III in this position, for example, on a buoy, a radio object-locating system transmitting with vertical polarization and receiving 4 1 with horizontal polarization or vice versa. would be greatly sensitive to signals returned by the dlplane since the'polarlzation of the incident energy is shifted by 90 on reflection. Sea return signals, or so called ground or sea clutter, shift the polarization of the reflected energy a relatively small amount and hence would affect the receiving apparatus a negligible amount. Even in rough seas or in heavily wooded or built-up areas a diplane reflector as described provides a very outstandingmarker for navigation of ships or aircraft.

One important advantage of the diplane according to this invention is that signals returned by the diplane to a radio object-locating system can be readily distinguished from signals returned by other fixed or movable objects even though the latter signals may be ofequal or stronger intensity than those returned by the diplane. This is achieved by comparison of the returned signals on an indicator such as a cathode ray tube. For example, signals may be transmitted with vertical polarization and received by antennas adapted to receive both vertical and horizontal polarization. By known means, the returned signals may be passed through a comparison device, .the output strength of which will depend upon the excess of streng-th of signals of one polarity over those of the other, and then pass to the indicator. If the diplane is adapted to shift the polarization by 90, the returned signal therefrom will be horizontally polarized and may be made to appear as a strong positive signal on the indicator. On the other hand, signals returned by other objects will 'be vertically polarized or have a non-uniform he may have available, or alternatively they may be designed to efficiently reflect only one particular type of wave, and indeed, even only one particular range of wavelengths of a particular type of wave, if so desired, whereby only navigators employing the appropriate type of exploratory beam would detect the presence of the reflector. Alternatively a particular arrangement of selectively reiiecting devices can be employed to convey additional information as to location to navigators employing several types or even several wavelengths of a given type of exploratory beams.

The structure as hereinbefore described may therefore be employed to enable navigators tc determine whether they are on one vside or the other of a particular plane and by obvious arrangements of such reflectors, navigation ir. relatively narrow channels, such as the navigation of aircraft through mountain passes, can be made much less hazardous. Such reflectors can also obviously be disposed so las to enable aviators to determine their approximate altitudes when near the earth and for numerous other analogous purposes.

As will be understood diplane I0, with the two reflecting members I I and I2 meeting each other at right angles, will reflect back in the direction of approach, only such waves as impinge upon it from directions normal to the common axis or line I3. Such a diplane will obviously be valuable in enabling navigators to determine when they are vwithin the area delimited by particular y boundary pl-anes, the boundary planes obviously being at right angles to the aforementioned line of meeting of lmembers ll and l2 and located at the respective ends thereof. The "back surfaces of members Il and l2 are preferably made non-.reflecting or dispersive to avoid any possible confusion which might result from reflection of Waves which chanced to be normally incident to such -back surfaces.

It is possible that instances will arise in which combinations of dispersive and reflecting structures may be employed to advantage to provide greater contrast between areas from which it is intended that reflection be obtained and the surrounding areas. In some cases too, it may be more desirable to absorb in whlole, or in part, the incident energy beam rather than disperse it. Absorption Where desired may be attained by methods well known to the art and not directly related to the present invention.

The above are merely illustrative of preferred embodiments of the invention, and it will be understood that the invention is capable of various other modifications and improvements. Therefore, it is not desired that the scope of the invention be limited to the precise details set forth.

What is claimed is:

1. In an environment characterized by background objects that 4shift the polarization'of a plane polarized Wave a negligible amount, the method of locating a beacon reflector remotely positioned from a transmitter and disposed among said background objects, which method comprises transmitting a plane polarized wave of electromagnetic energy, shifting the plane of polarization of said energy through by means of said beacon, and detecting said energy so shifted in polarization.

2. In an environment characterized by background objects that shift the polarization of a plane polarized wave a negligible amount, the method of locating a beacon reflector remotely -positioned from a transmitter and disposed among said background objects, which method comprises transmitting a plane polarized wave of electromagnetic energy, continuously shifting the plane of polarization of said Wave by rotating said diplane beacon, and detecting said energy so shifted in polarization.

EMMETT L. HUDSPETH.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number 

